1. Field of the Invention
The present invention relates to a feedforward amplifier with linear, low distortion characteristics, which is used for satellite communications, terrestrial microwave communications, mobile communications and the like.
2. Description of Related Art
FIG. 12 is a block diagram showing a basic configuration of a first conventional feedforward amplifier disclosed in Japanese patent application laid-open No. 1-198809/1989. In FIG. 12, the reference numeral 1 designates a distortion detector, and 2 designates a distortion eliminator. The feedforward amplifier is mainly composed of the distortion detector 1 and distortion eliminator 2.
In the distortion detector 1, the reference numeral 3 designates a main amplifier signal path, 4 designates a first linear signal path, and 8 designates a directional coupler. In the main amplifier signal path 3, the reference numeral 9 designates a variable attenuator, 10 designates a variable phase shifter, 11 designates a main amplifier, and 12 designates a second pilot signal injector. In the first linear signal path 4, the reference numeral 17 designates a delay circuit.
In the distortion eliminator 2, the reference numeral 5 designates a main amplifier output path, 6 designates a distortion injection path, and 15 designates a directional coupler. In the main amplifier output path 5, the reference numeral 14 designates a delay circuit, and in the distortion injection path 6, the reference numeral 19 designates a directional coupler, 20 designates a variable attenuator, 21 designates a variable phase shifter and 22 designates a sub-amplifier.
The reference numeral 23 designates an input terminal of the feedforward amplifier, 24 designates an output terminal, and 16 designates a directional coupler connected between the distortion eliminator 2 and the output terminal 24.
The reference numeral 7 designates a first pilot signal injector for injecting to an input path a first pilot signal of a particular frequency, which is generated by a signal source 25; and 26 designates a signal source for injecting to the second pilot signal injector 12 a second pilot signal of a particular frequency. The reference numeral 27 designates a level detector that detects the level of the first pilot signal by observing the output level of the directional coupler 19 interposed in the distortion injection path 6; and 28 designates another level detector that detects the level of the second pilot signal by observing the output level of the directional coupler 16.
The reference numeral 29 designates a controller that controls the electrically controllable variable attenuator 9 and variable phase shifter 10 such that the output level of the level detector 27 becomes minimum, and controls the electrically controllable variable attenuator 20 and variable phase shifter 21 such that the output level of the level detector 28 becomes minimum.
Next, the operation of the first conventional feedforward amplifier will be described.
In the distortion detector 1, the input signal applied to the input terminal 23 and the first pilot signal of the particular frequency, which is generated by the signal source 25 and injected through the first pilot signal injector 7, are delivered to the main amplifier signal path 3 and the first linear signal path 4 by the directional coupler 8. The signal supplied to the main amplifier signal path 3 passes through the variable attenuator 9 and variable phase shifter 10, and is amplified by the main amplifier 11. The signal supplied to the first linear signal path 4 passes through the delay circuit 17 with a delay time identical to that of the main amplifier signal path 3.
The two signals passing through the two signal paths are supplied through the directional couplers 13 and 18 to the distortion injection path 6 of the distortion eliminator 2. Thus, the directional coupler 19 in the distortion injection path 6 and the level detector 27 detect the first pilot signal, and the controller 29 controls the variable attenuator 9 and variable phase shifter 10 such that the output level of the level detector 27 is kept minimum. This means that the variable attenuator 9 and variable phase shifter 10 are controlled such that the two signals are combined in the same amplitude and opposite phases in the distortion injection path 6 of the distortion eliminator 2 through the directional couplers 13 and 18.
Since the signal from the main amplifier signal path 3 consists of a linearly amplified signal plus nonlinear distortion components of the main amplifier 11, and the signal from the first linear signal path 4 consists of only a linear signal, and the two signals are combined in the same amplitude and opposite phases, the distortion injection path 6 detects only the nonlinear distortion components due to the main amplifier 11.
In the distortion eliminator 2, the signal supplied to the main amplifier output path 5 is the sum of the linearly amplified signal through the main amplifier 11, the nonlinear distortion components, and the second pilot signal of the particular frequency injected by the second pilot signal injector 12, whereas the signal supplied to the distortion injection path 6 is the sum of the nonlinear distortion components of the main amplifier 11 and the second pilot signal.
The delay amount of the delay circuit 14 is set equal to the delay amount of the distortion injection path 6 in advance. The directional coupler 15 combines the two signals passing through the two paths 5 and 6, and the directional coupler 16 and the level detector 28 detect the level of the second pilot signal.
In this case, the controller 29 controls the variable attenuator 20 and variable phase shifter 21 such that the output level of the level detector 28 is kept minimum. This means that the variable attenuator 20 and variable phase shifter 21 are controlled such that the two signals are combined by the directional coupler 15 in the same amplitude and opposite phases. Thus, the nonlinear distortion components added through the main amplifier output path 5 and the nonlinear distortion components added through the distortion injection path 6 are cancelled out at the output terminal 24, so that only the linearly amplified signal by the main amplifier 11 is produced from the output terminal 24.
As a result, controlling the variable attenuators 9 and 20, and the variable phase shifters 10 and 21 makes it possible to limit the degradation in the distortion characteristics of the feedforward amplifier mainly due to the characteristic changes in the main amplifier 11 and sub-amplifier 22 by variations in temperature and a supply voltage.
FIG. 13 is a block diagram showing a fundamental configuration of a second conventional feedforward amplifier disclosed in Japanese patent application laid-open No. 5-235790/1993. The feedforward amplifier employs, instead of the first pilot signal in the distortion detector 1 of the first conventional feedforward amplifier, one carrier selected from the input signal consisting a plurality of modulation signals.
In mobile communications, mobile stations search for a carrier of a control channel called a perch channel, which is sent from a base station in order to locate their own zone and communicate with the base station that provides a maximum receiving field. The base station normally transmits at a fixed maximum transmission power through the control channel, and hence its level does not change as the carriers of other communication channels.
In FIG. 13, reference numerals 30.sub.1 -30.sub.n each designate a modulator producing one of modulation signals F1-Fn obtained by modulating carriers of different frequencies f1-fn. Here, it is assumed that the modulation signal Fm output from the m-th modulator 30.sub.m is assigned to the control channel described above.
The reference numeral 31 designates a modulation signal combiner that combines the modulation signals F1-Fn, and supplies the combined signal to the feedforward amplifier, in which the directional coupler 19 detects the modulation signal Fm of the control channel, and the controller 29 controls the variable attenuator 9 and variable phase shifter 10 such that the output level of the level detector 27 is kept minimum.
Thus, the second conventional feedforward amplifier can achieve the same effect as the first conventional feedforward amplifier by utilizing one of the carries in the input signal consisting of the plurality of the modulation signals instead of the first pilot signal in the distortion detector 1 in the first conventional feedforward amplifier.
FIG. 14 is a block diagram showing a basic configuration of a third conventional feedforward amplifier disclosed in Japanese patent application laid-open No. 4-364602/1992.
Generally speaking, the feedforward amplifier consists of two interference circuits for signal cancellation and distortion cancellation, each of which can be represented by a model of a feedforward interference circuit as shown in FIG. 14. The feedforward interference circuit represents the distortion detector 1 or distortion eliminator 2 in the first conventional feedforward amplifier as shown in FIG. 12.
It comprises the directional coupler 8 for splitting the input signal supplied through the input terminal 23 into two power signals; two signal transfer paths 53 and 54 to which the two distributed signals are supplied; and the directional coupler 15 that combines power signals from the signal transfer paths 53 and 54 to be output through the output terminal 24. The signal transfer paths 53 includes the variable attenuator 9, the variable phase shifter 10 and an amplifier 55, and the signal transfer path 54 includes the delay circuit 17 and a phase inverter 40. Here, the amplifier 55 corresponds to the main amplifier 11 in the distortion detector 1, or to the sub-amplifier 22 in the distortion eliminator 2.
The first pilot signal fed from the signal source 25 is spectrum spread by a modulator 41 and is supplied to the first pilot signal injector 7. The modulator 41 comprises a phase modulator 43 and a band-pass filter 44. A demodulator 42 comprises a band-pass filter 45, a local oscillator 46, a phase modulator 47, a mixer 48 and a pilot band-pass filter 49. In addition, a pseudorandom (PN) generator 51 supplies the modulator 41 and demodulator 42 with PN signals, respectively, which are synchronized by a phase modulator 52.
Next, the operation of the third conventional feedforward amplifier will be described. The first pilot signal of a single frequency from the signal source 25 is phase modulated and spectrum spread by the phase modulator 43 using the PN code supplied from the pseudorandom (PN) generator 51, and the spread signal is injected to the input signal by the first pilot signal injector 7. The output of the first pilot signal injector 7 is supplied through the directional coupler 8 to the signal transfer paths 53 and 54 whose outputs are combined by the directional coupler 15. The directional coupler 16 extracts part of the output of the directional coupler 15.
The first pilot signal is demodulated by the demodulator 42 using the output of the phase modulator 47 that phase modulates and spectrum spreads the output of the local oscillator 46 using the PN code supplied from the pseudorandom (PN) generator 51. The level detector 28 detects the level of the demodulated first pilot signal. The controller 29 controls the variable attenuator 9 and variable phase shifter 10 such that the detection level of the first pilot signal is kept minimum.
With the foregoing arrangements, the conventional feedforward amplifiers use pilot signals for controlling the variable attenuator 9 or 20, and the variable phase shifter 10 or 21 in the distortion detector 1 or distortion eliminator 2.
The arrangements, however, have a problem of degrading the communication quality because the detection level of the pilot signals is minimum at the optimum operation point, and hence the power level of the pilot signal to be injected must be increased to heighten the detection sensitivity. In particular, when using the pilot signal in the distortion detector 1, the distortion eliminator 2 cannot remove the first pilot signal completely, leaving part of it in the output signal. Thus, a new circuit must be added to eliminate the first pilot signal, which presents a problem of increasing the circuit scale.